Typically, such turbines include an exhaust structure which comprises an outer exhaust casing in the form of an annulus, and an inner exhaust tunnel defined by an annular exhaust cylinder or shield with an annular dead air space defined between the outer casing and the inner shield. Simply stated, an outer casing or cylinder surrounds an inner shield or cylinder in the exhaust area of a turbine.
The turbine typically includes a cylindrical bearing housing disposed around a central axis of a turbine and located and centered there, preferably by two sets or arrays of radially-extending struts. The respective struts in each array, at about 120 degrees apart, are encased in strut housings, extending radially inwardly to the bearing housing from the outer exhaust casing and through the inner shield to the bearing housing. The strut housings are welded to the inner shield, where they pass outwardly therethrough to the outer casing and inwardly therethrough to the bearing housing. Typically the struts, strut housings and inner shield are made from high heat-resistant materials or alloys, including, for example, an alloy known in the industry as “hastalloy”.
It is in the area of the welded joinery between the strut housings and the inner shield which suffer the principal consequences of differential thermal-caused material expansion and contraction.
As the turbine is operated, it can produce exhaust gases of high temperatures such as 1000 to 1300 degrees Fahrenheit or more. This heat, applied to the strut housing and to the inner shield causes them to expand or move in respective directions in response to this heating. Thus, the strut housing may expand longitudinally (in a radial direction from the bearing housing) while the inner shield moves or expands in other directions, or at least at different rates, all in response to the exhaust heat. Thus, there is a differential of material movement and destructive force at the welded joint between the strut housing and the inner shield. This differential causes the inner shield to flex, then crack or break apart at the intersection area at and around the welded joint to the strut housing. The dead air space between the inner shield and outer casing is thus opened to direct exhaust gas.
In the past, this material failure is cured only by shutting down the turbine, accessing the cracked area, removing the affected parts of the inner shield and welding replacement and reinforcing plates in the shield and to the strut housing. This work is expensive, requires periods of turbine shutdown, is difficult to access, is subject eventually to repeat of the continuing problem and is, for these and other reasons, very problematical.
Such turbines generally experience these adverse thermally-caused movement and force differentials on startup from inoperative conditions. Those cycles occur periodically on even a daily basis or multiple times per day. Thus, every time the turbine is started, the thermal expansion produces the described stress and flexes the inner shield eventually to failure.
Accordingly, it has been one objective of this invention to provide an improved exhaust structure for turbines which is not subject to the failure of materials due to differential thermal expansion and/or contraction.
A further objective of the invention has been to provide an improved exhaust structure for a turbine extending the life and maintenance requirements of such turbines in the exhaust components.
A further objective of the invention has been to provide an improved method for handling turbine exhaust.